Modern liquid toner electrostatic imaging began with the invention of a new class of toners referred to herein as ElectroInk® (which is a trademark of Indigo, N.V. of The Netherlands). This toner is characterized by its comprising toner particles dispersed in a carrier liquid, where the toner particles are comprised of a core of a polymer with fibrous extensions extending from the core. When the toner particles are dispersed in the carrier liquid in a low concentration, the particles remain separate. When the toner develops an electrostatic image the concentration of toner particles increases and the fibrous extensions interlock. A large number of patents and patent applications are directed toward this type of toner and charge directors which are comprised in it. These include: U.S. Pat. Nos. 4,794,651; 4,842,974; 5,047,306; 5,407,307; 5,192,638; 5,208,130; 5,225,306; 5,264,312; 5,266,435; 5,286,593; 5,300,390; 5,346,796; 5,407,771; 5,554;476; 5,655,194; 5,792,584 and 5,5923,929, PCT Patent publication WO 92/17823 (equivalent to U.S. patent application Ser. No. 08/203,596, now U.S. Pat. No. 6,146,803) and PCT patent application PCT/IL99/00394, the disclosures of all of which are incorporated herein by reference.
It has been discovered that this type of toner allows for high quality offset printing at high speed. However, this type of printing is described inter alia in patents and patent application Ser. Nos. 4,678,317; 4,860,924; 4,980,259; 4,985,732; 5,028,964; 5,034,778; 5,047;808; 5,078,504; 5,117,263; 5,148,222; 5,157,238; 5,166,734; 5,208,130; 5,231,454; 5,255,058; 5,266,435; 5,268,687; 5,270,776; 5,276,492; 5,278,615; 5,280,326; 5,286,948; 5,289,238; 5,315,321; 5,335,054; 5,337,131; 5,376,491; 5,380,611; 5,426,491; 5,436,706; 5,497,222; 5,508,790; 5,527,652; 5,552,875; 5,555,185; 5,557,376; 5,558,970; 5,570,193; 5,571,645; 5,572,274; 5,585,900; 5,592,269; 5,596,396; 5,610,694; 5,636,349; 5,655,194; 5,737,666; 5,745,829; 5,749,032; 5,793,490; 5,854,960; 5,864,353; 5,900,003; 5,915,152; 5,923,929; 5,935,754 and PCT publications (now U.S. patent applications) WO 96/29633 (08/930,249); WO 96/35182 (08/945,415); WO 96/29633 (08/894,707); WO 97/07433 (09/011,634); WO 97/39385 (09/171,396); WO 98/55901 (09/445,035) and WO 99/454,433 and PCT applications PCT/IL98/00553 and PCT/IL99/00363, the disclosures of all of which are incorporated herein by reference. Systems incorporating various ones of these patents are sold under the names E-Print 1000®, Omnius®, Turbostream™ and Cardpress™.
In the course of time various methods for development of latent electrostatic images have been developed for liquid toner, as described in the above referenced patents and patent applications. In addition, the speed of printing has increased. Printing is performed on a large number of materials. Other operating conditions have also changed.
In general, ElectroInk comprises a polymer or polymers (usually pigmented) which solvate the carrier liquid at some temperature above room temperature (and preferably above normal storage temperatures of 30–40° C.) and do not solvate the carrier liquid or dissolve substantial amounts of it below that temperature. Above the solvation temperature the polymer adsorbs the carrier liquid and is plasticized and softened by it. At elevated temperatures the toner material is thus soft enough to bond with a paper substrate. In practice, the temperature and pressure at which transfer to paper is made is controlled so that the transfer is complete, the transferred toner is fixed to the paper and the image is not squashed.
PCT publication WO 92/17823 (U.S. patent application Ser. No. 08/203,596) discloses a toner comprising two components in which the viscosity vs. temperature characteristics are specified. In particular, it was found that toner based on a mixture of polymers having a break-point on a semi-logarithmic plot of viscosity vs. temperature at between 3×105 and 5×106 at a temperature of below 65° C., when in a 40% by weight of toner polymer mixed with carrier liquid, allowed for good transfer and fixing of the image at relatively low transfer blanket temperatures. This weight percentage was chosen as a standard since it represented a proportion believed to exist during transfer of the toner to a paper substrate.